Created attachment 764725[details]
screenshot
In the contacts sidebar in compose window, the column titles of the found cards list are white.
It is caused by 3 { background-color: transparent; } style in
chrome://messenger/skin/addressbook/abContactsPanel.css applying to these elements.
Can you check if those are intentional (e.g. so that the panel can be overlaid on different backgrounds) or just a mistake?
This was found on Win XP but please check other OSes too.

Created attachment 765477[details][diff][review]
patch
You are correct, this is only visible on classic Windows where the treecols are affected by such stylings. On all other systems the -moz-appearance: treeheadercell hides the custom stylings.
I've also removed unneeded rules which are already in addressbook.css and included with the @import on top of abContactsPanel.css.

Comment on attachment 774142[details][diff][review]
patch unbitrotted
[Approval Request Comment]
This is only a visual bug and could help TB 24ESR to look better but this was a bug which was in since a long time without any user reaction, so it makes nothing when it can't land on ESR.